Electromagnetically actuated microshutter

ABSTRACT

The invention relates to an electromagnetically actuated microshutter comprising: a moveable plate that can rotate about an axis, connected to a stationary frame by two arms aligned on both sides of the plate to said axis, and comprising on its periphery a conductive loop; and below the assembly formed by the stationary frame and the moveable plate, a group of magnets having distinct magnetic orientations, arranged in such a manner so as to create, in regard to the moveable plate, a lateral magnetic field, in the plane of the frame, oblique in relation to the axis of rotation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national stage application under 35 U.S.C. §371of International Application No. PCT/FR2010/052301 and claims thebenefit of Int'l Application No. PCT/FR2010/052301, filed Oct. 27, 2010and French Application No. 09/57543, filed Oct. 27, 2009, the entiredisclosures of which are incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

The present invention relates to moveable microshutters formed byproduction methods for micro-electromechanical systems (MEMS). Inparticular, it pertains to a new electromagnetically actuatedmicroshutter structure. An example of application of the presentinvention relates to electromagnetically actuated micro-mirrors.

DESCRIPTION OF PRIOR ART

Moveable micro-mirrors based on MEMS technology are used in numerousdevices, for example, in miniaturized projection systems, and in visibleor infrared light sensors, such as bar code readers. Of interest hereare micro-mirrors attached to a frame by an axis and orientatable aroundthis axis by electromagnetic means.

FIG. 1 schematically shows an MEMS structure, formed in a silicon wafer,including an electromagnetically actuated moveable micro-mirror. Thisstructure comprises a reflective, moveable small plate 1, ormicro-mirror, attached to a stationary frame 3. A gap 5 extends betweenmoveable plate 1 and frame 3. Plate 1 is connected to frame 3 by twoarms 7 and 9 aligned on both sides of the plate, along a same axis 11.Thus, plate 1 is rotatable around axis 11 formed by arms 7 and 9. Themovement of plate 1 exerts torsion on arms 7 and 9.

A conductive path 13 follows the periphery of the front face of plate 1.Path 13 passes across arm 7 and ends in contacts 15 and 17 formed onframe 3. Contacts 15 and 17 are suited to be connected to a powersource, which is not shown, in such a manner that a current flows inconductive path 13 in the direction represented by arrows 19 (in thecase of direct current).

The assembly of frame 3 and plate 1 are subjected to a lateral magneticfield, represented by arrows 21, wherein the field lines aresubstantially perpendicular to axis 11 and substantially parallel to theplane of frame 3.

When a current flows through conductive path 13, opposite Laplace forcesare exerted orthogonally to the plane of the frame, on the portions ofpath 13 parallel to axis 11, and in which opposite currents flow. Thesecombined forces produce a rotation of plate 1 around its axis 11, of anangle determined in particular by the direction and intensity of thecurrent. Therefore, it is possible to modulate the orientation andinclination of plate 1 by varying the sign and value of the voltageapplied between contacts 15 and 17.

FIG. 2 shows a cross-sectional view that schematically represents anMEMS structure including an electromagnetically actuated micro-mirror ofthe type described in relation to FIG. 1. In this view, one can see thatthe silicon wafer is hollowed out below the location where moveableplate 1 is formed. A support 23 closes off this recess. Also generallyprovided above the micro-mirror is a cover, which is not shown and ispreferably transparent to protect plate 1 from the intrusion ofcontaminants.

Two magnets 25 and 27 are placed symmetrically on both sides of axis 11.Magnets 25 and 27, having a lateral magnetic orientation create, in thearea of mobile plate 1, a magnetic field whose field lines areorthogonal to axis 11 and parallel to the plane of frame 3.

A disadvantage of micro-mirror structures of the type described inrelation to FIGS. 1 and 2 lies in the size associated with the placementof magnets 25 and 27.

In practice, moveable plate 1 may be a square measuring about 1 mm on aside, gap 5 may measure substantially 50 μm, and the frame may have awidth of substantially 1 mm.

The surface area (as seen from above) of magnets 25 and 27 is added tothe surface area of frame 3. And yet, magnets 25 and 27 are relativelydistant from moveable plate 1. Therefore, to ensure a sufficient fieldin the area of moveable plate 1, they must have dimensions on the orderof 2 mm wide, 2 mm thick, and 3 mm long. The total surface area of thestructure is therefore at least doubled by the presence of magnets 25and 27.

FIG. 3 schematically shows another MEMS structure including anelectromagnetically actuated micro-mirror. For clarity's sake, only thedifferences in relation to FIG. 2 shall be detailed.

To limit the size, it is proposed to replace magnets 25 and 27 having alateral magnetic orientation in FIG. 2 by a magnet 31 having a verticalmagnetic orientation, placed under the assembly formed by moveable plate1 and frame 3. Magnet 31 creates, in regard to the moveable plate, amagnetic field whose field lines are orthogonal to the plane of frame 3.

A conductive coil divided into two separate windings 33 and 35 of theopposite direction, arranged on the front face of plate 1, on the sideof each of the edges of plate 1 that are parallel to axis of rotation11, respectively, is provided. This coil is suitable to be connected toa power source.

When a current flows in the coil, opposing attractive and repellingforces are exerted between magnet 31 and each of the windings 33 and 35,resulting in a rotational movement of moveable plate 1 around its ownaxis 11.

A disadvantage of this type of micro-mirrors lies in the loss of usablesurface on the front face of moveable plate 1, which is associated withthe dimensions of windings 33 and 35. The mass of the moveable part willalso be greater, which requires one to provide, for a given magneticfield, higher currents to result in its displacement. In addition,because of its increased mass, the moveable plate will be less resistantto impacts and accelerations.

SUMMARY

Therefore, an object of an embodiment of the present invention is topropose an electromagnetically actuated microshutter structure thatcompensates for all or some of the disadvantages of conventionalstructures.

An object of an embodiment of the present invention is to propose such astructure that has a small surface dimension.

An object of an embodiment of the present invention is to propose such astructure that is simple to produce.

Therefore, an embodiment of the present invention provides for anelectromagnetically actuated microshutter comprising: a plate that isrotatable about an axis, connected to a stationary frame by two armsaligned on both sides of the plate along said axis, and comprising onits periphery a conductive loop; and under the assembly formed by thestationary frame and the moveable plate, a group of magnets of distinctmagnetic orientations arranged in such a manner as to create, for themoveable plate, a lateral magnetic field, in the plane of the frame,that is oblique in relation to the axis of rotation.

According to an embodiment of the present invention, from a top-downview, the dimensions of the group of magnets are strictly equal to thedimensions of the assembly formed by the stationary frame and themoveable plate.

According to an embodiment of the present invention, the lateralmagnetic field is orthogonal to the axis of rotation.

According to an embodiment of the present invention, the group ofmagnets comprises three magnets in the shape, as seen from the top, ofbars parallel to the axis of rotation, juxtaposed in the same planeparallel to the plane of the stationary frame, the central magnet havingthe same width as the moveable plate and having a lateral magneticorientation; and the peripheral magnets having magnetic orientationsorthogonal to the plane of the frame and in the opposite direction.

According to an embodiment of the present invention, the extremities ofthe conductive loop cross one of the arms and are connected to contactelements formed on the stationary frame.

According to an embodiment of the present invention, the upper surfaceof the moveable plate is reflective.

According to an embodiment of the present invention, the microshutterhas a protective cover above the stationary frame.

According to an embodiment of the present invention, the moveable plateis connected to the stationary frame by means of a moveable frameconnected to the stationary frame by two arms aligned on both sides ofthe moveable frame relative to a secondary axis of rotation that isorthogonal to said axis, the moveable frame comprising a conductiveloop.

According to an embodiment of the present invention, the lateralmagnetic field is substantially 45 degrees in relation to the axis ofrotation.

BRIEF DESCRIPTION OF THE DRAWINGS

These objects, characteristics, and advantages, as well as others, shallbe explained in detail in the following description of specificembodiments, given by way of non-limiting example in relation to theenclosed drawings, wherein:

FIG. 1, described earlier, schematically shows a structure including anelectromagnetically actuated micro-mirror.

FIG. 2, described earlier, is a cross-sectional view of a structure ofthe type described in relation to FIG. 1.

FIG. 3, described earlier, is a cross-sectional view schematicallyshowing another structure including an electromagnetically actuatedmicro-mirror.

FIG. 4 is a cross-sectional view schematically showing a samplestructure including an electromagnetically actuated micro-mirror.

FIG. 5 is a cross-sectional view schematically showing a samplestructure including an electromagnetically actuated micro-mirroraccording to an embodiment of the present invention.

FIG. 6A is a schematic top view of a structure including anelectromagnetically actuated micro-mirror according to a design variantof the present invention;

FIG. 6B is a top, cross-sectional view of FIG. 6; and

FIG. 7 is a perspective view showing another example of an applicationof a part of the structure described in relation to FIG. 5.

DETAILED DESCRIPTION

For clarity's sake, identical elements were designated by the samereferences in the different drawings and, in addition, as is common inshowing micro-components, the various figures are not drawn to scale.

FIG. 4 is a cross-sectional view schematically showing a samplestructure including an electromagnetically actuated micro-mirror, formedin a silicon wafer. Like the structure described in relation to FIG. 2,this structure comprises a reflective, moveable plate 1, fixed in astationary frame 3. A gap 5 separates moveable plate 1 from frame 3.Moveable plate 1 is connected to frame 3 by two, or pairs of arms, notshown, aligned on both sides of the plate along the same axis 11. Thus,plate 1 is rotatable about axis 11.

A conductive loop 13 follows the periphery of the front face of moveableplate 1. The extremities of path 13 cross, for example, over one of themounting arms of plate 1 and end in contacts, not shown, formed on frame3 and suitable for being connected to a power source.

A magnet 41, having a lateral magnetic orientation, is placed under theassembly formed by moveable plate 1 and frame 3. Such a magnet creates,in regard to moveable plate 1, a magnetic field substantially orthogonalto axis of rotation 11. This field is connected to the return fieldlines from one of the magnet's poles to the other, and its direction,shown by arrow 43, is substantially opposite to the magnetic orientationof magnet 41.

When a current flows through conductive path 13, opposite Laplace forcesare exerted on portions of path 13 that are parallel to axis 11 and inwhich opposite currents flow. This causes a rotation of moveable plate 1about its own axis 11, according to an angle determined by the directionand intensity of the current.

However, such a structure is purely theoretical. In fact, the magneticfield created by the magnet is distributed all around it, and withrespect to the moveable plate, the field is insufficient to obtain asignificant displacement of plate 1. It would require a high-intensitycurrent moving through loop 13 to obtain a significant displacement ofthe moveable plate, which would result in an excessive consumption ofenergy.

FIG. 5 is a cross-sectional view schematically showing a structureincluding an electromagnetically actuated micro-mirror according to anembodiment of the present invention. For clarity's sake, only thedifferences with the structure shown in FIG. 4 will be detailed here.

Instead of magnet 41 with its lateral magnetic orientation, one providesfor a group of three magnets having distinct magnetic orientations,arranged in such a manner as to create, in regard to moveable plate 1, amagnetic field parallel to the plane of frame 3 and orthogonal to theaxis of rotation 11, sufficient for obtaining a displacement of moveableplate 1 without an excessive consumption of energy.

According to a preferred embodiment, three magnets 51, 53, 55 having,from a top-down view, the shape of bars parallel to axis 11, juxtaposedin a same plane parallel to frame 3 are provided. Magnet 51 hassubstantially the same width as the moveable plate 1, for example, onthe order of 1 mm, and substantially the same length as the assemblyformed by frame 3 and plate 1, for example of the order of 3 mm. Thegroup of 3 magnets has substantially the same width as frame 3. It is ofcourse understood that these dimensions are given solely for examplepurposes. The width of central magnet 51 may be slightly less, forexample, or slightly greater than the width of moveable plate 1.Preferably, to limit the size, the dimensions of the group of magnetswill be strictly equal or slightly less than those of the assemblyformed by the stationary frame and the moveable plate. However, in thepreceding, it is understood that “substantially the same width” refersto widths equal to substantially plus or minus 30%.

Central magnet 51 has a lateral magnetic orientation, in other words anorientation that is substantially parallel to the plane of frame 3 andsubstantially orthogonal to axis 11.

Peripheral magnets 53 and 55 have a vertical magnetic orientation, inother words an orientation that is substantially orthogonal to the planeof frame 3. The magnetic orientations of magnets 53 and 55 aresubstantially opposite in direction.

Thus, for moveable plate 1 and above the central part of the magnetgroup, field lines created by each of the magnets 51, 53, 55 aresubstantially lateral and in the same direction. These elements add upto create a lateral magnetic field, represented by arrow 57, sufficientto obtain a displacement of moveable plate 1 with a decreasedconsumption of energy. However, in regard to the magnet group oppositemoveable plate 1, the field lines have a tendency to cancel each otherout. This makes it possible in particular to limit the electromagneticinteractions with other elements of a device.

It is of course to be understood that the direction of the magneticorientations of the peripheral magnets and the central magnet areselected in a suitable manner so that the lateral components of thefield lines add up on the side of the moveable plate and cancel eachother out on the bottom side of the magnets, and not the other wayaround.

FIGS. 6A and 6B schematically show a variant design of the presentinvention. They depict an MEMS structure comprising anelectromagnetically actuated microshutter in which the microshutter ismoveable relative to two distinct axes of rotation, for exampleorthogonal ones.

FIG. 6A is a top-down view. A moveable plate 61 is connected to amoveable frame 63, itself connected to a stationary frame 65. Two gaps67 and 69 extend respectively between moveable plate 61 and moveableframe 63, and between moveable frame 63 and stationary frame 65. Plate61 is connected to moveable frame 63 by two arms 71 a and 71 b alignedon both sides of the plate along an identical axis 72. Thus, plate 61 isrotatable about axis 72. Moveable frame 63 is connected to stationaryframe 65 by two arms 73 a and 73 b aligned on both sides of the moveableframe along an identical axis 74, for example orthogonal to axis 72.Thus, moveable frame 63 is rotatable about axis 74. Plate 61 istherefore moveable, via moveable frame 63, relative to the twoorthogonal axes of rotation 72 and 74.

As in the examples described in relation to FIGS. 1, 2, 4, and 5, aconductive path 76 follows the periphery of the front face of moveableplate 61. The extremities of path 76 (not shown) run across one of themounting arms of plate 61 and across one of the mounting arms ofmoveable frame 63, and end, on frame 65, in contacts that are suited forconnecting to a power source.

In addition, a conductive path 78 follows the periphery of the frontface of moveable frame 63. The extremities of path 78 (not shown) runacross one of the mounting arms of the moveable frame and end, on frame65, in contacts suited for being connected to a power source.

The assembly consisting of frame 65, moveable frame 63, and plate 61 issubjected to a magnetic field represented by arrows 80, substantiallyparallel to the plane of the frame, and substantially oriented at 45°relative to the axes of rotation 72 and 74.

It is possible to individually actuate each of the axes 72 and 74 byvarying the sign and value of the current applied in each of the paths76 and 78. The rotating movement of plate 61 in relation to axis 72 isassociated with the component of magnetic field 80 that is orthogonal toaxis 72. The movement of moveable frame 63 in relation to axis 74 isassociated with the component of magnetic field 80 that is orthogonal toaxis 74.

FIG. 6B is a cross-sectional, top-down view of the MEMS structure ofFIG. 6A, schematically representing an group of three magnets havingdistinct magnetic orientations, arranged under the assembly, shown inFIG. 6A, formed by moveable plate 61, moveable frame 63, and frame 65.This group of magnets is suitable for creating, for plate 61 andmoveable frame 63, magnetic field 80 that is substantially parallel tothe plane of frame 65 and substantially at 45° in relation to axes ofrotation 72 and 74.

Similar to the embodiment described in relation to FIG. 5, there areprovided three magnets 81, 83, 85 having, in a top-down view, the shapeof bars orthogonal to the direction of field 80, juxtaposed in a sameplane parallel to frame 65. The bars forming magnets 81, 83, 85 arepreferably cut in such a manner that the group of magnets does notextend past the assembly consisting of frame 65, moveable frame 63 andplate 61.

Central magnet 81 has a lateral magnetic orientation, substantiallyparallel to the plane of frame 65 and substantially at 45° in relationto the axes of rotation 72 and 74. Peripheral magnets 83 and 85 have avertical magnetic orientation, substantially orthogonal to the plane offrame 65. The magnetic orientations of magnets 83 and 85 have ansubstantially opposite direction.

In regard to plate 61 and moveable frame 63, the field lines created byeach of the magnets 81, 83, and 85 are substantially parallel to theplane of frame 65, substantially at 45° relative to axes 72 and 74, andsubstantially of the same direction. These elements add up to create themagnetic field shown in FIG. 6A by arrows 80.

An advantage of the proposed embodiments is that the magnets arearranged under the assembly formed by the moveable plate and the frameand they do not extend past this assembly. This enables one to reduce byat least a factor of two the surface area of a micro-mirror in relationto a conventional structure of the type shown in FIG. 2. In addition,the absence of magnets having thicknesses greater than that of thesilicon, on both sides of the frame, allows one, in the case of amicro-mirror, to increase the angles of incidence of the light rays.

A particular advantage of the structure shown in FIGS. 6A and 6B isthat, when the mirror is placed on one of its sides, it is oriented tohorizontally and vertically reflect a beam, which corresponds to thedeflection directions most often desired in actual practice.

In addition, in the proposed embodiments, the magnetic field created inregard to the moveable plate is sufficient to obtain displacements ofthe plate with a decreased consumption of electricity. This isparticularly associated with the fact that the magnets are placed veryclose to the conductive paths in comparison to conventional structures.To further reduce the distance between the magnets and conductive paths,one can use a thin silicon wafer.

Furthermore, the movements of the plate are controlled by the flow ofcurrent in a simple loop arranged at the periphery of the plate.Accordingly, the losses of useful surface area on the plate, associatedwith the conductive paths, are minimized. More generally, thethree-magnet structure described above has the advantages of having asmall size and creating a strong lateral magnetic field in a localizedspace.

FIG. 7 shows another example of an application of a three-magnetstructure of the type described above. In this example, the group ofmagnets is used to interact with magnetic microbeads or with cellssuspended in a biological analysis device.

This device comprises a channel 90 defined by partitions. Under channel90, there are arranged three magnets 91, 93, and 95, having distinctmagnetic orientations, so as to create, in regard to the channel, alateral magnetic field. In this example, the magnetic orientations ofthe magnets, represented in the drawing by arrows, are identical tothose of the three-magnet assembly of FIG. 5.

In channel 90, there flows a fluid comprising magnetic microbeads 97(occasionally described in technological terms by “super-para-magneticbeads”), for example beads comprising iron oxide particles and whosediameter is between 50 nm and 3 μm. Particles suited for capturingbiological targets (molecules, cells, viruses, etc.) may have first beengrafted on beads 97. Provisions may be made for different types of beadsand/or different types of grafted particles, suitable for capturingdifferent type of biological targets.

The magnetic field and the gradient of the magnetic field to which issubjected channel 90 allow one to trap the beads to separate those thathave captured a biological target from those that have not. This enablesone, for example, to measure the concentration of the biological targetin question in the fluid. In addition, the gradient of the magneticfield allows one to separate the beads that have captured differentbiological targets.

Furthermore, in the fluid, cells may also be circulating that, due totheir diamagnetic or paramagnetic properties, are drawn or repulseddirectly by the field and the field gradient generated by the structureof magnets. One could then dispense with beads 97 to act on these cells.

Particular embodiments of the present invention have been described.Diverse variants and modifications will become apparent to one havingordinary skill in the art. In particular, the invention is not limitedto the application described above, namely moveable micro-mirrors. Onecan also implement the sought after functioning on any device comprisingelectromagnetically actuated moveable microshutters. One can also adaptthe invention to other types of devices comprising shutters, membranes,or other moveable structures, for example acceleration sensors,gyroscopes, pressure sensors, and microphones, based on the principle ofelectromagnetic actuation or motion detection using an electromagneticfield.

In addition and in relation to FIGS. 5 and 6B preferred embodiments ofthe present invention have been described that comprise a central magnethaving a lateral magnetic orientation and two peripheral magnets havingvertical magnetic orientations in the opposite direction. One havingordinary skill in the art will know how to implement the sought afterfunctioning by using other configurations.

Furthermore, the embodiments described in relation to FIGS. 5 and 6A, 6Bprovide for two peripheral magnets with a vertical magnetic orientation.To optimize the intensity of the lateral magnetic field in regard to themoveable plate, one can, if necessary, use peripheral magnets having aslightly oblique magnetic orientation in relation to the vertical.Similarly, the central magnet may be split in two halves of a magnethaving magnetic orientations that are slightly oblique in relation tothe plane of the frame and oblique in relation to each other. In athree-magnet assembly, one can also provide for magnets having differentwidths so as to optimize the lateral field in regard to the moveableplate.

What is claimed is:
 1. An electromagnetically actuated microshuttercomprising: a moveable plate that can rotate about an axis, connected toa stationary frame by two arms aligned on both sides of the plateaccording to said axis, and comprising on its periphery a conductiveloop; and below the assembly formed by the stationary frame and themoveable plate, a group of magnets having distinct magneticorientations, arranged in such a manner so as to create, in regard tothe moveable plate, a lateral magnetic field, in the plane of the frame,orthogonal to the axis of rotation, wherein said group of magnetscomprises, three magnets in the form, when seen from above, of barsparallel to the axis of rotation, juxtaposed in a same plane parallel tothe plane of the stationary frame, the central magnet having a widthequal to that of the moveable plate and a lateral magnetic orientation;and the peripheral magnets having magnetic orientations that areorthogonal to the plane of the frame and of the opposite direction, thedimensions of the group of magnets being, when seen from above, equal tothe dimensions of the assembly formed by the stationary frame and themoveable plate.
 2. A microshutter according to claim 1, wherein saidframe is rectangular and said arms extend from the middle of twoopposite sides of the rectangle.
 3. A microshutter according to claim 2,wherein the moveable plate is connected to the stationary frame by meansof a moveable frame connected to the stationary frame by two second armsaligned on both sides of the moveable frame according to a secondaryaxis of rotation orthogonal to said axis, the moveable frame comprisinga conductive loop, the magnetic field being substantially at 45 degreesin relation to the axis of rotation.
 4. A microshutter according toclaim 3, wherein the central magnet extends diagonally in a directionorthogonal to the field and the peripheral magnets are located in thecorners of the structure left free by the central magnet.
 5. Amicroshutter according to claim 1, wherein the extremities of theconductive loop run across one of said arms, and are connected tocontact elements formed on the stationary frame.
 6. A microshutteraccording to claim 1, wherein the upper surface of the moveable plate isreflective.
 7. A microshutter according to claim 1, further comprising aprotective cover disposed above the stationary frame.